Field of the Invention
The present invention relates to compositions and methods for control of microbial growth, for example, in oilfield compositions. The present invention also relates to microbicides, and more particularly, to the use of biocides in gas and oil field well fluids, for example, fluids used in drilling operations, stimulation operations, and/or post stimulation activities such as produced and flowback waters.
Description of Related Art
The production of oil from oilfields involves several phases. Most of these phases can be affected with unwanted microbial activity. Also, in other industries, there can be problems of microbial contamination. There is an ongoing need for improved methods and compositions for controlling these undesired microorganisms.
After a well is drilled into a subterranean geological formation that contains oil, natural gas, and water, every effort is made to maximize the production of the oil and/or gas. To increase the permeability and flow of the oil and/or gas to the surface, the drilled wells are often subjected to well stimulation.
Well stimulation generally refers to several post drilling processes used to clean the well bore, enlarge channels, and increase pore space in the interval to be injected thus making it possible for fluids to move more readily into the formation.
A typical well treatment process generally includes pumping specially engineered fluids at high pressure and rate into the subterranean geological formation. The high-pressure fluid (usually water with some specialty friction reducing fluid additives to reduce pumping pressure and maximize rock fracturing) exceeds the rock strength and opens a fracture in the formation, which can extend out into the geological formation for as much as several hundred feet. Certain commonly used fracturing treatments generally comprise a carrier fluid (usually water or brine) and a polymer, which is also commonly referred to as a friction reducer. Many well stimulation fluids will further comprise a proppant. Other compositions used as fracturing fluids include water with additives, viscoelastic surfactant gels, gelled oils, crosslinkers, oxygen scavengers, and the like.
The well treatment fluid can be prepared by blending the polymer with an aqueous solution (sometimes an oil-based or a multi-phase fluid is desirable); often, the polymer is a solvatable polysaccharide. The purpose of the polymer is generally to decrease the turbulent frictional forces of the fracturing fluid that aids in the creation of a fracture; and to sufficiently thicken the aqueous solution so that solid particles of proppant can be suspended in the solution for delivery into the fracture.
The polymers used in well fluids are subjected to an environment conducive to bacterial growth and oxidative degradation. The growth of the bacteria on polymers used in such fluids can materially alter the physical characteristics of the fluids. For example, bacterial action can degrade the polymer, leading to loss of viscosity and subsequent ineffectiveness of the fluids and, more importantly, lead to the plugging of the fracture (due to biofilms growth) and subsequent reduction in recovery of the desired hydrocarbons. Fluids that are especially susceptible to bacterial degradation include those that contain polysaccharide and/or synthetic polymers such as polyacrylamides, polyglycosans, carboxyalkyl ethers, and the like. In addition to bacterial degradation, these polymers are susceptible to oxidative degradation in the presence of free oxygen. The degradation can be directly caused by free oxygen or mediated by aerobic microorganisms. Thus, for example, polyacrylamides are known to degrade to smaller molecular fragments in the presence of free oxygen. Because of this, microbicides and oxygen scavengers are frequently added to the well treatment fluid to control bacterial growth and oxygen degradation, respectively. Desirably, the microbicide is selected to have minimal or no interaction with any of the components in the well stimulation fluid.
For example, the microbicide should not affect fluid viscosity to any significant extent and should not affect the performance of oxygen scavengers, often derived from bisulfite salts, contained within the fluid. However, a reduced viscosity can be overcome by additional polymer and/or crosslinkinker addition. This situation is more desirable than an increased viscosity.
Other desirable properties for the microbicide are (a) cost effectiveness, e.g., cost per liter, cost per square meter treated, and cost per year; (b) safety, e.g., personnel risk assessment (for instance, toxic gases or physical contact), neutralization requirements, registration, discharge to environment, and persistence; (c) compatibility with system fluids, e.g., solubility, partition coefficient, pH, presence of hydrogen sulfide, temperature, hardness, presence of metal ions or sulfates, level of total dissolved solids; (d) compatibility with other treatment chemicals, e.g., corrosion inhibitors, scale inhibitors, demulsifiers, water clarifiers, well stimulation chemicals, and polymers; and (e) handling, e.g., corrosiveness to metals and elastomers, freeze point, thermal stability, and separation of components.
Current well stimulation fluids often employ either glutaraldehyde, or tetra-kis-hydroxymethyly-phosphonium sulfate (THPS), or 2,2-dibromo-3-nitrilopropionamide (DBNPA), or other fast acting biocides to control bacterial contamination.
A common industry practice is to control microbial growth by adding an effective amount of a quick kill biocides followed by controlled sequential addition of another relatively slower acting biocide. Illustrative of quick-kill biocides include alkanedials for example lower alkanedials such as C1-C8 alkane dials such as propanedial, butanedial, pentanedial, hexanedial, and the like. Preferred is DBNPA and pentanedial (glutaraldehyde).
It is also well known within the industry to use a quick kill biocide (such as glutaraldehyde), followed within minutes, hours, or days, by a relatively slower acting biocide to reduce or inhibit microbial activity. For instance, EP0337624B 1 teaches a method of controlling oilfield biofouling, comprising adding an effective amount of quick kill biocide selected from one or more alkanedials, for example C3-C7 alkanedials, and then adding, by controlled sequential addition, an effective amount of isothiazolone, which functions as a slower acting biocide.
Glutaraldehyde (pentanedial) can be problematic to use because it is hazardous to handle and has environmental concerns. Moreover, it has been reported in the literature that glutaraldehyde can deleteriously affect the fluid viscosity of the well treatment fluid at elevated temperatures; temperatures that are commonly observed during use of the well treatment fluid. This can be problematic in fracturing applications since the higher maintained fluid viscosity down hole could hinder flow back. In addition, glutaraldehyde has been shown to negatively impact the behavior of the oxygen scavenger.
With regard to THPS, although it has been shown to perform better than glutaraldehyde with respect to interaction with the oxygen scavengers, THPS has been found to interact with the polymer and limit viscosity development when added pre-inversion and post-inversion. That is, THPS has been observed to interact with the polymer during shear and significantly reduce fluid viscosity.
Thus, there remains a need for a more versatile microbiocide for use, for example, in oil and gas wells, that can effectively control bacterial contamination and have minimal interaction with the polymer and/or oxygen scavenger. The present invention addresses these and other needs.